Exaggerated scarring, or fibrosis, complicates a spectrum of diseases, occurs in all organs, and can be relentless, debilitating, and deadly. It is particularly prevalent in older patients and thoe who have experienced military exposures, posing a serious problem for veterans. Exaggerated innate and adaptive immune responses drive fibrosis, yet the exact mechanisms remain incompletely understood. Recent findings suggest a role for interleukin (IL)-33 in the pathogenesis of fibrosis. IL-33 exists in two forms and has a dual activity. Intranuclear IL-33 (INIL33) is bound to chromatin and appears to regulate gene expression. By contrast, extracellular IL-33 (EXIL33) acts by binding to the cell-surface receptor T1/ST2 and facilitating Th2 responses. The activities of EXIL33 are under active investigation, whereas the activities of INIL33 have been underappreciated and not studied in detail or in association with disease. Our data suggest that fibrosis is driven primarily (yet not exclusively) by INIL33. The levels of INIL3 are elevated in tissues of patients with fibrosis as well as in the mouse model of bleomycin injury, in which the INIL33 form predominates over EXIL. Gene delivery of INIL33 in cell culture and in the animal model leads to collagen accumulation, increased production of profibrotic cytokines and matrix metalloproteinases, as well as Smad3 activation, all of which are known to contribute to fibrosis. When combined with bleomycin injury, INIL33 potentiates collagen accumulation and the expression of profibrotic cytokines. There is minimal, if any, conversion of INIL33 to EXIL33, no signs of Th2 activation, and gene deficiency of T1/ST2 has minimal, if any, attenuating effect on INIL33-driven fibrosis. We hypothesize that INIL33 is a T1/ST2-independent, Th2-independent, upstream activator of multiple profibrotic mechanisms and that elevated expression of INIL33 in fibroblasts contributes to collagen deposition by these cells through direct and indirect mechanisms. The following Specific Aims will be addressed: 1. Functionally map the IL-33 molecule to identify regions in this protein that confer nuclear localization, chromatin binding, and transcriptional regulation in cultured primary human fibroblasts and in mice in vivo. 2. Define the relative roles of direct mechanisms (binding to the collagen promoter and upregulating collagen gene transcription) and indirect mechanisms (both T1/ST2-dependent and -independent regulation of profibrotic cytokines and matrix metalloproteinases) of profibrotic regulation of fibroblasts by IL-33 in cell culture. 3. Delineate the pathophysiological roles of fibroblast-specific expression of IL-33 and of the T1/ST2 cell-surface receptor in vivo by determining the effects of fibroblast-specific IL-33 deficiency, ubiquitous IL-33 deficiency, and ubiquitous T1/ST2 deficiency on inflammation and fibrosis in the bleomycin injury model. Successful completion of these studies will clarify the role and mechanisms of fibroblast activation by INIL-33, laying the groundwork for future development of rational IL-33-targeting antifibrotic strategies.